The invention relates to a semiconductor diode laser of the index-guided type, often referred to as laser for short hereinafter, comprising a semiconductor body with a semiconductor substrate of a first conductivity type on which a semiconductor layer structure is disposed which comprises at least in that order a first cladding layer of the first conductivity type, an active layer, and a second cladding layer of a second conductivity type opposed to the first, and comprising a pn junction which can generate coherent electromagnetic radiation, given a sufficient current strength in the forward direction, in a strip-shaped active region situated within a resonance cavity which is limited by surfaces extending substantially perpendicular to the active region, while the semiconductor layer structure is provided with means for forming a step in the effective refractive index on either side of the active region, and the first and the second cladding layer are provided with further means for forming an electrical connection. The invention also relates to a method of manufacturing such a laser.
Such a laser has various applications as a radiation source: optical disc systems, optical glass fibre communication systems, bar code readers, and laser printers. Lasers of the index-guided type are attractive especially because the emerging beam is diffraction-limited and the far field and wave front change comparatively little with the supplied optical power, and thus with the current through the laser, in sharp contrast to a laser of the gain-guided type. Lasers of a weakly index-guided type, in addition, are comparatively easy to manufacture. In many of the applications mentioned, furthermore, a laser is desired which can supply a maximum optical power, i.e. the highest possible yield of electromagnetic radiation.
Such a laser with a strip-shaped geometry is known from "Heterostructure Lasers, Part B: Materials and Operating Characteristics" by H. C. Casey and M. B. Panish, Academic Press 1978, Ch. 7.6, pp. 207-217. The diode presented therein, for example in FIGS. 7-6-5(a), comprises an n-GaAs substrate with an active layer of GaAs disposed thereon between an n-type and a p-type AlGaAs cladding layer. The electrical connection means comprise a metal layer at the substrate side and a p-GaAs contact layer and a further metal layer at the side of the upper cladding layer. The further means comprise the presence of a mesa above the active region which occupies a major portion of the second cladding layer, so that the laser is of the (weakly) index-guided type.
A disadvantage of the known laser is that it is not capable of supplying a very high useful power. The so-called P (=optical power) versus I (=electric current) characteristic does not show one substantially linear gradient above the threshold current --as is desirable--, but a kink is often found in practice in said P-I characteristic at a comparatively low optical power. At such a kink, the derivative of the optical power versus the current strength changes, and the emitted radiation beam is no longer diffraction-limited. It will be clear that such an effect limits the usefulness of the laser to optical powers below the optical power where such a kink occurs. The effect described above will be referred to hereinafter as kinking. The optical power at which such a kink is observed will be referred to as the kink power (P.sub.kink).